Disc pack couplings are commonly used to connect a driving shaft, from a power output apparatus such as a motor or a turbine, to a shaft to be driven, which is connected to a power input apparatus such as a pump or a generator.
It is not always possible to construct or position the power output apparatus and the power input apparatus so that the driving shaft (the shaft being driven) and driven shaft (the shaft being driven) are exactly aligned. In any case, the running of the machines, wear and tear, and environmental factors may later the relative positions of the driving shaft and driven shaft. The coupling between the driving shaft and driven shaft must be able to account for or compensate for such changes in alignment. In some applications, the coupling may also have to cope with sudden changes of rotation, and the direction of rotation changing.
A common means of doing this is the use of disc pack couplings. A spacer shaft is located between the driven shaft and the driving shaft. Disc packs are placed between the driven shaft and the spacer shaft to join them, and between the driving shaft and the spacer shaft. Each disc pack comprises a plurality of annular plates. Each disc pack is attached to the two shafts it joins at alternating points around its body, so that the disc pack can flex between these alternating connections. The disc pack transfers torque, so that rotation of the driving shaft causes the spacer shaft to rotate, which in turn causes the driven shaft to rotate. The disc pack though can flex to accommodate an axial separation between the spacer shaft and the driven shaft, an angular misalignment, or a combination of these two things. The joint provided by the disc pack between the driving shaft and the spacer shaft flexes in the same manner.
Misalignment of the input and driving shafts may be characterised as angular (where the two shafts' axes are not parallel), radial (where the two shafts' axes are parallel but not co-axial) and axial (where the distance between the ends of the two shafts is incorrect), and in general a combination of each type of misalignment will be present.
The amount of misalignment that a single disc pack can accommodate, known as the maximum angular misalignment, is particularly influenced by the size of the disc pack and the torque that is to be transmitted. Typically though, a disc pack can accommodate ½° to 1½° of angular misalignment. This also determines the amount of radial misalignment that the two disc packs and spacer shaft can accommodate. If axial alignment must be accommodated, this reduces the angle that the disc pack can accommodate.